Electrochemical cells and capacitors typically include a container with an opening that is closed by a lid or cover welded to the container to form a casing for the electrical energy storage device. Inside the container is an anode/cathode electrode assembly activated by an electrolyte. The container and the lid are of electrically conductive material and serve as a contact for either the anode electrode or the cathode electrode. In a case negative cell, the anode current collector is in contact with the casing while for a case positive design, the opposite is true. The other of the anode electrode and the cathode electrode not in contact with the casing is connected to a terminal lead electrically insulated from the casing by a glass-to-metal seal. When a load is connected to the casing and the terminal pin in an electrochemical cell, a chemical reaction in the cell results in a voltage differential that generates an electrical current to power the load, for example, a medical device.
The lid must provide access to the interior of the casing for at least two purposes. First, the terminal lead connected to the anode or the cathode current collector must pass through one of the lid openings to a position exterior of the casing. Second, the electrolyte must be filled into the housing through the other lid opening. Conventionally, two openings are defined in the lid for this purpose. The openings usually have structures connected to the lid to aid in sealing them. For example, a terminal lead ferrule is attached to the lid to accommodate the electrical lead and a fillport/closure assembly is used for sealing the fill opening.
FIG. 1 shows an exemplary prior art construction where the lid 10 is formed of a generally rectangular blank 12 stamped from a sheet of electrically conductive material. During stamping, two openings 14, 16 are provided through the blank 12. A terminal lead ferrule 18 and a fillport 20 are sleeve-shaped members formed of discrete parts that are welded to the blank 12, each in registration with one of the openings 14, 16.
This prior art lid requires a number of manufacturing, inspection, and assembly steps due to the use of at least three discrete parts, i.e. the blank 12, the terminal lead ferrule 18 and the fillport 20. Specifically, the blank 12 is punched from a sheet of metal using a fine blanking or stamping operation. Simultaneously, the two openings 14, 16 are punched through the blank 12. The lid 10 goes through an annealing process, a passivation process (e.g. removal of free iron from the surface of the part) and a cleaning process before it is inspected. The discrete terminal lead ferrule 18 and the discrete fillport 20 go through the same process steps prior to attachment to the blank 12. The terminal lead ferrule 18 and the fillport 20 are then positioned in registration with the openings 14, 16 and welded thereto. These welds are vulnerable to variations in quality and each must be inspected. As those who are skilled in the art will readily recognize, these manufacturing, assembly, and inspection steps require time and labor. Also, inventory of the parts must be tracked and maintained, further adding to the cost of an electrical energy storage device.
Another problem with the prior art cover construction is that crevice corrosion can occur where the terminal lead ferrule 18 and the fillport 20 are secured to the openings 14, 16 in the lid 10. Typically, the terminal lead ferrule 18 and the fillport 20 are inserted from the bottom or interior surface of the blank 12 before being welded. This welding may leave cracks or crevices between the mating surfaces leading to entrapment of materials such as cleaning solutions. As such, corrosion can occur around these crevices.
One prior art lid or cover for an electrochemical cell is described in U.S. Pat. No. 6,010,803 to Heller, Jr. et al. This lid is formed by a metal injection molding process which requires that the intersections between the terminal lead ferrule and the main body of the lid and between the fillport structure and the lid be slightly curved or “radiused.” Heller, Jr. et al. believes that radiused junctions facilitate the flow of material during the metal injection molding process. This eliminates areas of stress concentration that can cause the molded material to crack.
There are several problems with the Heller, Jr. et al. metal injection-molded lid. First, the radiused areas detract from the internal volume available to active and other no-active cell components. Electrical energy storage devices of the present invention are used in implantable medical devices such as cochlear implants. These are extremely small devices that require extremely compact power sources where maximizing internal volume is important.
Secondly, a machined lid according to the present invention has a higher density and, consequently, less porosity than the metal injection molded lid. Metal injection molded materials require a binder, and even though technology advances have reduced the amount of binder required, metal density is still about 98% to about 98.5%, after curing. In contrast, a one piece lid according to the present invention machined from bar or rod stock has a density of about 99.99%, and maintains acceptable mechanical properties required for glassing the terminal lead in the glass-to-metal seal.
Lastly, design structures can be repositioned or changed to accommodate a particular electrical energy storage device. Typically, this requires a program change to offset features, change tolerances or add new features. In contrast, metal injection molded components require a whole new set of tooling which is capitol intensive.
U.S. Pat. No. 5,173,375 to Cretzmeyer et al. related to a unitary lid for closing a casing for an electrochemical cell. This prior art lid is stamped from a metal blank. The problem is that the stamping process introduces minute stress fracture into the product lid. These are sights of potential seal failure and corrosion.
Accordingly, what is needed is a unitary lid having a terminal ferrule and fillport structure that reduces the manufacturing, assembly, and inspection steps described above and is as compact in size as possible.